I always did this in the past with my older iPhones and a 3rd Gen Apple TV: launch a facetime session, enable AirPlay mirroring to the TV and turn the phone sideways to get landscape mode so that the image would take up the entire TV screen.
My wife now has an iPhone XS and has been trying to do a landscape FaceTime with me while I'm away and it never fills the 46" HDTV. Yes, it rotates when she flips the phone, but it only rotates within the narrow vertical space, i.e. as if someone was to take a vertical video and display it on a widescreen TV.. massive unused space on left and right of image.
I've made sure that the "lock rotate" button is not enabled, but I can't find any reason why it isn't rotating into a landscape view and taking up the whole TV screen.
Any thoughts? Did Apple remove this feature for some reason?
EDIT: I found this link which someone seems to suggest it still works, but I can't find elsewhere on the internet anyone claiming it works.